Method and apparatus for measuring radiation in a borehole

ABSTRACT

Apparatus and methods for measuring radiation in a borehole environment using a YAlO 3 :Ce (YAP) scintillation crystal. Borehole instruments are disclosed which employ a gamma ray detector comprising a YAP scintillator coupled to a light sensing means such as a photomultiplier tube. One instrument embodiment combines a YAP scintillation detector and a source of pulsed neutrons. Borehole environs are irradiated with neutrons, and induced gamma radiation is measured using a YAP scintillation detector. Response of the detector is used to determine characteristics of the borehole environs. Mechanical and physical properties of YAP are utilized to obtain improved measurements. The relatively short light decay constant of YAP minimized pulse pile-up in the detector when measurements require that the detector be operated during a neutron pulse.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] This disclosure is related to radiation measurements usingscintillation type radiation detectors, and more specifically related toapparatus and methods for measuring radiation in a borehole environmentusing a YAlO₃:Ce (YAP) scintillation crystal.

[0003] 2. Background of the Invention

[0004] Scintillation type radiation detectors have been used for decadesin a wide variety of applications. Radiation absorbed by a scintillationcrystal emits a pulse of light or “scintillates”. The intensity of lightis a function of energy deposited within the crystal by the absorbedradiation. A measure of light intensity can, therefore, be related tothe energy of radiation absorbed by the scintillator. A measure of thenumber of scintillations per unit time can be related to the intensityof radiation absorbed by the scintillation crystal.

[0005] In fabricating a scintillation type radiation detector, ascintillation crystal is optically coupled to a light sensitive devicethat responds to the number and to the intensity of scintillationsproduced within the crystal. Phomultiplier tubes (PMT) are commonly usedas light sensitive devices. A PMT converts scintillations from thecoupled crystal into electrical pulses. A pulse is typically generatedfor each scintillation. The magnitude of the pulse is proportional tothe intensity of the scintillation. A count per unit time of pulses can,therefore, be related to the intensity of radiation impinging upon thecrystal. Measures of magnitudes of the pulses can, therefore, be relatedto corresponding energies of the radiation absorbed by the crystal.Alternately, scintillation crystals can be optically coupled to othertypes of light sensitive devices such as photodiodes, and intensity andenergy of impinging radiation can be determined from electrical outputsof these devices.

[0006] The scintillation process is not instantaneous and, in fact, thescintillation emission intensity follows an exponential decay. Thalliumactivated sodium iodide, or NaI(Tl), is a commonly used material inscintillation type gamma radiation detectors. The decay constant of ascintillation produced within a NaI(Tl) crystal by impinging gammaradiation is about 230 nanoseconds (ns). If the intensity of radiationimpinging upon the crystal is sufficiently intense to generate asubsequent scintillation pulse before the previous scintillation pulsehas decayed to a negligible level, the scintillation pulses willessentially “sum” within the crystal. This is commonly referred to aspulse “pile-up”. As an example, two pulses of equal intensity (inducedby two gamma rays of equal energy) which pile-up within a detectorsystem will produce a single electrical pulse output with a magnitudegreater than a pulse that would be produced by a single gamma ray. Sincepulse magnitude is related to radiation energy, pulse pile-up typicallyresults in an erroneous radiation energy measurement. Furthermore, sincethe pulses “sum” as a single rather than a multiple radiation detectorevents, pulse pile-up results in erroneous radiation intensitymeasurements in high intensity gamma ray fluxes. It is, therefore,highly desirable to utilize a scintillation crystal with a minimum lightdecay constant when measuring energy and intensity of high intensitygamma radiation fluxes. As an example, there is a class of boreholeinstruments that employs a source of pulsed neutrons and one or morescintillation detectors. Certain measurements, such as inelastic scattergamma ray measurements, require that the one or more detectors beoperated during the neutron burst. This exposes the one or moredetectors to extremely high fluxes of gamma ray and other types ofradiation. The light decay constant of the scintillation material is,therefore, a critical design parameter in this type of instrumentation.Many measurement systems using scintillation type gamma ray detectorsare also exposed to neutron fluxes. As in the example above, a largevariety of borehole instruments used to measure properties of earthformation penetrated by the borehole employ one or more scintillationgamma ray detectors and a neutron source. The neutron source, whetherpulsed or continuous, induces gamma radiation within the formationthrough several types of reactions including inelastic scatter andthermal capture. This induced gamma radiation is sensed by the one ormore scintillation detectors and is used to determine formation andborehole parameters of interest. The scintillation detectors are alsoexposed to neutrons from the source, and especially to thermal neutronsgenerated in the borehole environs. These neutrons can produceradiation-emitting isotopes within the scintillation crystal. This iscommonly referred to as crystal “activation”. Consider, as an example ascintillation detector comprising a NaI(Tl) crystal. The thermal neutroncapture cross sections for the primary elemental constituents sodium(Na) and iodine (I) are 0.43 barns and 6.15 barns, respectively. Thermalneutrons impinging upon the Nal (Tl) detector produce ²⁴Na and ¹²⁸Iwithin the scintillator through the ²³Na(n,γ)²⁴Na and ¹²⁷I(n, γ)¹²⁸Ireactions, respectively. Both ²⁴Na and ¹²⁸I decay through beta emissionwith ¹²⁸I also decaying through electron capture. There is often gammaemission subsequent to the beta decay or electron capture. Theseradiations are generated within the NaI(Tl) crystal, and both the gammaand beta radiation induce scintillations within the crystal. Theseactivation induced radiations are considered as “noise” in themeasurement of formation properties using gamma radiation induced withinthe formation and borehole. It is, therefore, highly desirable to use ascintillation crystal with primary elemental constituents that do notreadily “activate” when used in a system which also utilizes a neutronsource.

[0007] There are other considerations in selecting a scintillationcrystal for borehole applications. The borehole environment is typicallyharsh in that pressures and temperatures are typically high. Boreholeinstruments are subjected to shock and vibrations as the instrument istypically conveyed within the borehole. Crystals such as NaI(Tl) arehighly susceptible to shock induced cleavage, which typically worsenswith constant vibration. Cleavage, in turn, results in deterioratingenergy resolution and efficiency. As mentioned previously, temperatureis usually elevated within a borehole, and typically varies with depth.In particular, variations in temperature can adversely affect crystalscintillation properties of a crystal which, in turn, can adverselyaffect subsequent radiation energy and intensity measurements. Somescintillation crystals, such as NaI(Tl), are hygroscopic. This requiresthat the crystal be encased in a hermetically sealed container, whichincreases the overall dimensions of the crystal package for a givenactive crystal volume. This increase in size, or the resulting necessityto reduce the active volume of the crystal, can be a critical designfactor in borehole instrument fabrication. Inherent crystal gamma rayresolution properties and overall efficiency properties are also factorsin borehole logging instrument design.

[0008] Other scintillation crystals have been used in boreholeapplications. Typically, these scintillation materials exhibitadvantages over NaI(Tl) in some areas, but exhibit disadvantages inother areas. On such material is bismuth germinate (BGO), withproperties well documented in the literature.

[0009] The scintillation material cerium activated yttrium aluminumperovskite or YAlO₃:Ce (YAP) has a density of 5.55 grams per cubiccentimeter (g/cm3), an effective Z of 36, a light decay constant of 27ns, light output of 45% of NaI at 25° C., 18,000 photons/MeV, emissionpeak of 350 nanometer (nm), and an index of refraction of 1.94. Thermalneutron cross sections for the major constituents of the crystalyttrium, aluminum and oxygen are 1.28 barns, 0.230 barns and 0.00019barns, respectively. The activity produced by thermal neutron capture inyttruim is relatively long-lived so that decay radiation is negligiblecompared to that observed from iodine activation in NaI crystals.

[0010] YAP has been used in the prior art in a number of non-boreholescintillation detector applications, and especially in the field ofmedical imaging. Typical prior art applications are summarized below.

[0011] A gamma ray camera system comprising an array of YAP(Ce)scintillation crystals optically coupled to a position sensitivephotomultiplier tube is disclosed in “YAP Multi-Crystal Gamma CameraPrototype”, K. Blazek et al, IEEE Transactions on Nuclear Science, Vol.42, No. 5, October 1995. The multiple scintillation crystals areoptically isolated from one another.

[0012] A scintillator detector with multiple YAP crystals and othertypes of crystals is disclosed in “Blue Enhanced Large Area AvalanchePhotodiodes in Scintillation Detection with LSO, YAP and LuAP Crystals”,M. Moszynski et al, IEEE Transactions on Nuclear Science, Vol. 44, No.3, June 1997. Scintillator crystals are optically coupled to large areaavalanche photodiodes.

[0013] A high resolution positron emission tomograph (TierPET) forimaging small laboratory animals is discloses in “Recent Results of theTierPET Scanner”, S. Weber et al, IEEE Transactions on Nuclear Science,Vol. 47, No. 4, August 2000. The system is based on an array of YAPcrystals. 20×20 arrays of 2×2×15 mm polished YAP crystals are opticallycoupled to a position sensitive PMT.

[0014] U.S. Pat. No. 5,313,504 to John B. Czirr discloses the use of aYAP scintillator in a borehole instrument to monitor output of a neutronsource that is also disposed within the borehole instrument. Since theYAP scintillator is used in a neutron source monitor system, theinstrument is designed to maximize the response of the YAP scintillatorto the neutron source and, conversely, to minimize the response of theYAP scintillator to the borehole environs.

[0015] None of the above cited references discloses a system that issuitable for operation within a borehole to measure properties of theborehole environs.

SUMMARY OF THE INVENTION

[0016] The scintillation material YAlO₃:Ce (YAP) possesses manyproperties, as summarized above, which are ideally suited for use inscintillation type radiation detectors in borehole instrumentation. Morespecifically, YAP is rugged and less subject to shock and vibrationdamage when compared to other commonly used crystals such as NaI(Tl).YAP is not hygroscopic thereby eliminating the need of hermeticpackaging required for NaI(Tl) crystals. This increases designflexibility in borehole instrumentation. YAP is relatively high density(5.55 g/cm³), and is of similar efficiency to NaI(Tl) over theintegrated energy range of 0.1 to 9.5 MeV. The major constituents of YAPare less susceptible to thermal neutron activation than NaI(Tl). YAP isless susceptible to variation in temperature than NaI(Tl). Temperatureproperties of YAP are discussed in detail in “The Change of GammaEquivalent Energy with Temperature for scintillation DetectorAssemblies”, C. Rozsa, et al, Nuclear Science Symposium, 1999.Conference Record. 1999 IEEE, Volume: 2, 1999 Page(s): 686-690 vol.2.The change of relative light output of responses of YAP (Ce)scintillators to alpha and gamma radiation was investigated over thetemperature range—20 degrees Centigrade (°C.) to 70° C.

[0017] Probably the most significant characteristic of YAP, with respectto borehole instrumentation design, is the scintillation light decayconstant which is approximately ten times less than that of NaI(Tl).This reduces the problem of pulse pile-up in high intensity radiationfields. In non-pileup conditions, energy resolution of NaI(Tl) issomewhat better across the entire spectrum than the resolution of YAP.In high intensity fluxes, however, where pulse pile-up is a significantfactor in NaI(Tl) detectors, YAP detectors with significantly shorterlight decay constant exhibits superior energy resolution. Theseproperties are especially important in certain types of boreholeinstrumentation, which will be discussed in detail in subsequentsections of this disclosure.

[0018] Borehole logging instruments or “logging tools” can be embodiedin a variety of ways depending upon the desired borehole environsmeasurements. Tools detailed in this disclosure contain at least oneradiation detector comprising a YAP scintillation crystal opticallycoupled to a light sensing device such as a photomultiplier tube (PMT),a photodiode, or the like, which converts scintillation intensity to anelectrical pulse of proportional magnitude. The detector assembly ispreferably enclosed within a pressure housing for protection from theharsh borehole environment. The tool is conveyed along the borehole bymeans of a wireline, a drill string or a slick line.

[0019] Scintillation detector based formation evaluation instruments,whether wireline or logging-while-drilling (LWD) tools, can be embodiedto measure a wide variety of parameters. In one embodiment, the tool isused to measure only natural gamma radiation emitted by formationpenetrated by the borehole, or gamma radiation emitted by materials suchas radioactive “tagged” tracer materials within or in the immediatevicinity of the borehole. Other classes of formation evaluation toolscontain one or more sources of radiation, such as a neutron source,which induces a variety of reactions within the formation and borehole.Radiation produced by these reactions is typically measured by one ormore scintillation detectors within the tool. Formation and boreholeparameters of interest are then determined from the response of the oneor more detectors. The source of radiation within the tool can becontinuous or pulsed. Detectors are operated at specified times during apulsed radiation cycle to optimize the measure of radiation fromspecific reactions of interest.

[0020] This disclosure will be directed toward a YAP boreholescintillation detector embodied in a formation evaluation toolcomprising a pulsed source of 14 MeV neutrons. It should be understoodthat the YAP borehole scintillation detector can effectively be embodiedin borehole instruments containing other sources of radiation, andfurther embodied as a plurality of detectors. This disclosure willfurther be directed to a logging system wherein at least one YAPscintillation detector is operated during each neutron pulse therebyexposing the detector to an intense radiation flux. An additionalembodiment of the YAP scintillator operates the YAP scintillationdetector during the quiescent period between neutron pulses. Stillfurther embodiments operate the detector during the pulse and during thequiescent periods between pulses, or operate the detector without theuse of a source.

[0021] Either gross count rate or spectral energy count rates can bemeasured by the disclosed logging system for conversion into boreholeand formation parameters of interest. As an example, gamma radiationsensed by the YAP scintillation detector can be recorded in a pluralityof energy ranges or “windows” and these window count rates can berelated to specific reactions which, in turn, can be related toconcentrations of specific elements within the formation penetrated bythe borehole.

[0022] Other types of radiation, such as beta radiation, can generatescintillations within a YAP scintillation crystal. This disclosure will,however, be directed primarily to systems which involve the measurementof gamma radiation.

BRIEF DESCRIPTION OF THE DRAWINGS

[0023] So that the manner in which the above recited features,advantages and objects of the present invention are obtained and can beunderstood in detail, more particular description of the invention,briefly summarized above, may be had by reference to the embodimentsthereof which are illustrated in the appended drawings.

[0024]FIG. 1 depicts a YAP scintillation crystal and cooperating otherelements to form a YAP radiation detector deployed in a borehole;

[0025]FIG. 2 illustrates a logging tool utilizing a YAP radiationdetector and a source of radiation;

[0026]FIG. 3a is a pulsed neutron timing diagram for a thermal neutrondecay type logging tool;

[0027]FIG. 3b is a timing and response diagram of a YAP radiationdetector disposed in a thermal neutron decay type logging tool;

[0028]FIG. 4a is a pulsed neutron timing diagram for an inelasticscatter type logging tool;

[0029]FIG. 4b is a gamma ray production diagram for an inelastic scattertype logging tool;

[0030]FIG. 5a is an enlarged pulsed neutron timing diagram for aninelastic scatter type logging tool;

[0031]FIG. 5b is a corresponding timing and response diagram for aNaI(Tl) gamma ray detector disposed in an inelastic scatter gamma raytype logging tool;

[0032]FIG. 5c is a corresponding timing and response diagram for a YAPgamma ray detector disposed in an inelastic scatter gamma ray typelogging tool;

[0033]FIG. 6 illustrates the energy response of NaI(Tl) and YAPscintillation detectors in an intense flux of monoenergetic gammaradiation; and

[0034]FIG. 7 illustrates a gamma ray energy spectrum and selected energywindows used in an inelastic scatter type gamma ray logging system todetermine borehole and formation parameters of interest.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0035] A borehole logging tool comprising YAP radiation detectors can beembodied in a variety of ways depending upon the desired boreholeenvirons for the measurements. FIG. 1 illustrates a logging tool 11comprising a YAP scintillation crystal 12 optically coupled to a lightsensing device 14 such as a photomultiplier tube (PMT), a photodiode, orthe like, which converts scintillation intensity to an electrical pulseof proportional magnitude. The light sensing device 14 is typicallypowered and controlled by an electronic package 16. The electronicspackage 16 can also contain data processing equipment, such as circuitsto determine the intensity and energy of radiation impinging upon andinteracting with the scintillation crystal 12. The electronics package16 can also contain computing means to transform radiation energy andintensity into parameters of interest. The scintillation crystal 12,light sensing device 14 and electronics package 16 are enclosed within apressure housing 18 for protection from the harsh borehole environment.The tool is conveyed along a borehole 20 penetrating an earth formation22 by a conveyance system including a member 24 which extends from thetool 11 to a surface conveyance unit 26. If the conveyance system is awireline logging system, the member 24 is a wireline logging cable, andthe surface conveyance unit 26 comprises wireline draw works and surfaceequipment well known in the art. If the conveyance system is a drillingrig, the member 24 is a drill pipe string and the surface conveyance 26comprises a drilling rig, which is also well known in the art. Otherconveyance systems, such as a slickline system, can be used to conveythe tool 11 along the borehole 20. The surface conveyance unit 26 canalso contain data processing equipment, such as circuits to determinethe intensity and energy of radiation impinging upon and interactingwith the scintillation crystal 12. The surface conveyance unit 26 canalso contain computing means to transform radiation energy and intensityinto parameters of interest. Embodied as the tool 11, typicalmeasurements from the system would be naturally occurring gammaradiation emitted by the formation, or gamma radiation from radioactivetagged fluids and propants used in formation fracturing operations.

[0036]FIG. 2 illustrates a YAP radiation detector embodied as a logginginstrument 13 comprising a radiation source 42. The source can be anisotopic neutron source such as Americium-beryllium (Am-Be), a neutrongenerator, or any other type of radiation source, such as an isotopicgamma ray source or a high energy gamma ray source comprising anaccelerator. It will be assumed that the source 42 is a neutrongenerator, which is operated to produce pulses of neutron of energyaround 14 MeV. The source 42, along with a YAP scintillation crystal 32,an optically coupled light sensing device 34, and a controllingelectronics package 36 are all disposed within a pressure housing 38.Shielding material 40 is typically used to minimize direct irradiationof the crystal 32 by the source 42. It should be understood thatadditional YAP detectors can be used within the pressure housing 38 toenhance measurements or to obtain additional measurements of interest.

[0037] Still referring to FIG. 2, neutrons emitted by the source 42induce a variety of reactions within the formation and the boreholeenvirons. Radiation produced by these reactions are sensed by the YAPscintillation detector within the tool 13, and parameters of theformation 22 and the borehole 20 are determined from the response of theone or more detectors. The neutron source is pulsed for tool embodimentsdiscussed below. Pulse duration and pulse repetition rate parameters areadjusted to optimize the neutron induced reactions of interest.Likewise, the YAP detector is operated at specified times during apulsed radiation cycle to optimize the measure of radiation fromspecific reactions of interest.

[0038] The electronics package 36, and the surface conveyance unit 26,can contain data processing equipment, such as circuits to determine theintensity and energy of radiation impinging upon and interacting withthe scintillation crystal 32. The electronics package 36, and thesurface conveyance unit 26, can also contain computing means totransform radiation energy and intensity into parameters of interest.

[0039] The determination of formation saline water saturation from ameasure of the rate of thermal neutron capture was first introducedcommercially in the 1960s. This logging system is well known in theindustry under the generic name “thermal neutron decay” log and by avariety of service names. FIGS. 3a and 3 b illustrate conceptually theYAP detector embodied in a thermal neutron decay tool.

[0040]FIG. 3a is a plot of neutron source output N as a function oftime, and illustrates the neutron source pulse timing for the tool 13embodied as a thermal neutron decay type logging system. Referring toboth FIGS. 2 and 3, the source 42 is used to generate a sequence ofneutron pulses 50 of time duration 56. The pulses 50 are repeatedperiodically after a time interval 54, with a quiescent time 52 beingmeasured from the termination of a previous pulse to the initiation of asubsequent pulse. In typical formation and borehole conditions, thethermalization of fast neutrons from the 14 MeV source and subsequentcapture of thermal neutrons by elements within the formation occurs at arate with a half life of several hundred microseconds. The pulserepetition rate is typically about 1,000 pulses per second with a pulsewidth 56 of 50 to 100 μs. The quiescent period 52 is, thereforetypically 900 to 950 μs.

[0041]FIG. 3b is a plot 60 of the natural logarithm of gamma radiationintensity I measured as a function of time. During the time interval t₀to t₁, when the neutron source 42 is operating, composite gammaradiation is quite intense as can be seen from the magnitude of thecurve 60. This composite gamma radiation comprises gamma radiation frominelastic scatter reactions, and to a lesser extent gamma radiation fromthermal capture reactions, naturally occurring gamma radiation from theborehole environments and even a small component of neutron inducedactivation within the YAP crystal 32. Since the thermalization andcapture process is relatively slow, gamma radiation resulting primarilyfrom thermal neutron capture reactions are measured with the YAPdetector during at least two time intervals 62 and 64 occurring in thequiescent period 52 between neutron pulses 50. The detector is firstoperated starting at a time t₂ and ending at a time t₃ yielding a count66 as illustrated graphically by the shaded area. The detector is againoperated starting at a time t₄ and terminated at a time t₅ yielding acount 68, again as illustrated graphically by the shaded area. Radiationis typically not intense within these time intervals, therefore pulsepile-up is not a problem. The counts 66 and 68 are combined to obtainthe parameter of interest (saline water saturation) using methods wellknown in the art.

[0042] The determination of formation fresh water saturation from ameasure of gamma radiation resulting from neutron inelastic scatter wasfirst introduced commercially in the 1970s, and is generically known asthe “carbon/oxygen” “neutron inelastic scatter” log. FIGS. 4a and 4 billustrate the logging tool 13 embodied as an inelastic scatter typelogging system.

[0043] Attention is first directed to FIGS. 2 and 4a. FIG. 4a is a plotof neutron output N from the source 42 plotted as a function of time.Pulses 70 of 14 MeV neutrons from the source 42 induce inelastic scatterreactions within the formation 22 penetrated by the borehole 20. FIG. 4billustrates total gamma radiation flux for the source neutron output Nof FIG. 4a. Compared with the thermal neutron capture process, theinelastic scatter process is much faster and, in practice, isessentially instantaneous. As a result, measured radiation is veryintense during each neutron burst spanning the time interval t₀ to t₁ asillustrated in FIG. 4b. This radiation is primarily generated byinelastic scatter reactions. Because of the essentially instantaneousspeed of the inelastic scatter reactions, the YAP detector must beoperated during the neutron pulse within a time interval t₀ to t₁thereby exposing the YAP to very intense radiation. Pulse repetitionrate is typically 10,000 to 20,000 pulses per second since nomeasurements are made during the quiescent period between pulses 74. Therelatively low level of gamma radiation 75 shown between pulses 70typically comprises thermal capture radiation (capture componenttypically is larger than shown, relative to gammas during pulse),naturally occurring gamma radiation, and possibly very low levels ofactivation radiation from within the YAP crystal. Neutron pulse width 72is also reduced to about 5 μs to allow for the increased pulserepetition rate.

[0044] Pulse pile-up in a gamma ray detector in intense gamma radiationfields is a significant problem as discussed previously. This is thecase when the tool 13 is embodied as an inelastic scatter type toolbecause of the intense radiation flux in which the detector must operateduring a neutron pulse. The use of a YAP scintillation crystal, with itsrelatively short light decay constant, results in a significantlyimproved system when compared to prior art systems using a NaI(Tl)crystal with a light decay constant which is an order of magnitudegreater.

[0045]FIG. 5a illustrates enlarged views of three consecutive neutronpulses 70 as shown previously in FIG. 4a. The effects of pulse pile-up,and the minimization of this problem using a YAP scintillation crystal,will be illustrated with the following hypothetical example.

[0046] Attention is first directed to FIG. 5b. Assume that three gammarays of equal energy E_(i) impinge upon a NaI(Tl) detector during a timeinterval 72 during the first neutron pulse 70 at times 90, 91 and 92.FIG. 5b illustrates as a curve 82 voltage V(E_(γ)) generated by thelight sensing device optically coupled to the NaI(Tl) detector. Thevalue of V(E_(γ)) shown at 79 represents voltage representative of agamma ray of energy E_(i) if no pileup were present. Because of therelatively long light decay constant of NaI(Tl), the correspondingvoltage V(Eγ) from the gamma ray impinging at time 90 does not decay toa negligible level before the voltage buildup from the gamma rayimpinging at time 91. Voltage V(E_(γ)) from the gamma ray impinging attime 91 does not decay to a negligible level before the voltage buildupfrom the gamma ray impinging at time 92. The result is pulse pile-upthat produces a cumulative voltage pulse V(E_(γ)) of magnitude 80, whichis clearly greater that the value 79 that would be produced in theabsence of pile-up. Next assume that two gamma rays of energy E_(i)impinge upon the NaI(Tl) detector at times 93 and 94 during the timeinterval 72′. The time interval between the two gamma rays is less thatthe time interval between any of the impinging gamma rays from theprevious time interval 72. Pile-up is again a significant problemproducing a cumulative voltage pulse V(E_(γ)) of magnitude 80′, which isclearly greater that the value 79 that would be produced in the absenceof pile-up. Finally, assume that three gamma rays of energy E_(i)impinge upon the NaI(Tl) detector at times 95, 96 and 97 during the timeinterval 72″. The time interval spanned by the three gamma rays is lessthat the time interval spanned by the three impinging gamma rays fromthe previous time interval 72. Pile-up is even more significant than theprevious two examples, yielding a cumulative voltage pulse V(E_(γ)) ofmagnitude 80″ which is clearly greater that the pile-up values 80 and80′.

[0047] Attention is now directed to FIG. 5c. Assume that the three gammarays of equal energy E_(i) impinge upon a YAP detector during the timeinterval 72 of the first neutron pulse 70, again at the times 90, 91 and92. FIG. 5c illustrates as a curve 82′ the voltage V(E_(γ)) generated bythe light sensing device optically coupled to the YAP detector. Thevalue of V(E_(γ)) shown at 79 again represents voltage representative ofa gamma ray of energy E_(i) if no pileup is present. Because of therelatively short light decay constant of YAP, the corresponding voltageV(E_(γ)) from the gamma ray impinging at time 90 does decay to anegligible level before the voltage buildup from the gamma ray impingingat time 91. Voltage V(E_(γ)) from the gamma ray impinging at time 91does decay to a negligible level before the voltage buildup from thegamma ray impinging at time 92. This results in three well resolvedpulses which produce separate voltage pulses V(E_(γ)) of magnitude 79corresponding to three gamma rays of E_(γ). Stated another way, there isno pulse pile-up. Next consider the two gamma rays of energy E_(i)impinging upon the YAP detector at times 93 and 94 during the timeinterval 72′. As stated previously, the time interval between the twogamma rays is less that the time interval between any of the impinginggamma rays from the previous time interval 72. The YAP detector systemis still able to properly resolve the two gamma rays and generate thecorrect voltage pulses V(E_(γ)) of magnitude 79. . Finally, againconsider the three gamma rays of energy E_(i) which impinge upon the YAPdetector at times 95, 96 and 97 during the time interval 72″. Althoughthe time interval spanned by the three gamma rays is less that the timeinterval spanned by the three impinging gamma rays from the previoustime interval 72, the YAP detector system is still able to properlyresolve the three gamma rays and generate the correct voltage pulsesV(E_(γ)) of magnitude 79. Again, there is no pulse pile-up in the YAPcrystal. For the three hypothetical examples, pulse pile-up iseliminated using the YAP detector system.

[0048]FIG. 6 is a gamma ray energy spectrum consisting of a plot ofmeasured gamma ray intensity C_(γ) as a function of gamma ray energyE_(γ). Curve 102 represents a spectrum from the hypothetical exampleusing the NaI(Tl) detector system illustrated in FIG. 4b. The curve,which was induced by monoenergetic gamma radiation of energy E_(i), doesnot peak sharply at E_(i), but is significantly broadened to the highenergy side by pulse pile-up. Curve 100 represents a spectrum from thehypothetical example using the YAP detector system illustrated in FIG.4c. Since no pulse pile-up is present in the YAP detector, the spectrumis peaked sharply at energy E_(i).

[0049] The examples discussed above and illustrated in FIGS. 5a-5 c andFIG. 6 clearly illustrate the advantages of a YAP detector system inborehole applications, especially in high intensity gamma ray fluxfields. The light responsive means 14 and 34, and the electronicpackages 16 and 36 (see FIGS. 1 and 2), are designed to efficientlyprocess the scintillations generated by YAP scintillation crystals. Inborehole instrumentation, the light responsive means is typically aphotomultiplier tube. The PMT is selected with dynode string toeffectively process scintillation output pulses with short light decayconstants. Pulses are typically preamplified by circuitry in thecooperating electronics package. Preferably a charge integratingpreamplifier is used, wherein the preamplifier outputs electrical pulseswith rise and decay times commensurate with the short light constantpulses generated by the YAP scintillator. Proper selection of lightresponsive means and the use of complementary “fast” pre-amplificationcircuitry yields a detector assembly which efficiently processesscintillations with short light decay times. This efficient processingminimizes pulse pile-up in the detector assembly.

[0050]FIG. 7 illustrates a typical gamma ray spectrum measured with thetool 13 configured to detect inelastic scatter radiation. Typicallymeasured counts C_(γ) are integrated over preselected energy ranges or“windows” to obtain counts needed to determine formation and boreholeparameters of interest. As an example, four energy windows W₁, W₂, W₃and W₄ are shown at 112, 114, 116 and 118, respectively. Correspondingintegrated counts C₁, C₂, C₃ and C₄ are shown at 122, 124, 126 and 128,respectively, as represented by shaded areas. The windows W₁, W₂, W₃ andW₄ (and thus corresponding counts C₁, C₂, C₃ and C₄) might containradiation from inelastic scatter of neutrons from oxygen, carbon,calcium and silicon nuclei. These count rates can then be combined toobtain measures of fresh water formation saturation using methods wellknown in the industry.

[0051] As mentioned previously, the YAP scintillation crystal possesesmany properties which are ideally suited for borehole instrumentation.YAP is rugged and less subject to shock and vibration damage whencompared to other commonly used crystals such as NaI(Tl). YAP is nothygroscopic thereby eliminating the need of hermetic packaging requiredfor NaI(Tl) crystals, and thereby increasing design flexibility inborehole instrumentation. YAP is relatively high density (5.55 g/cm³),and is similar in efficiency in the detection of gamma radiation toNaI(Tl) over the integrated energy range of 0.1 to 9.5 MeV. The majorconstituents of YAP are less susceptible to thermal neutron activationthan the major constituents of NaI(Tl). YAP is less susceptible tovariation in temperature than NaI(Tl). In addition to measuring gammaradiation from reactions in the formation and borehole, YAP can be usedin conjunction with the borehole environs measurement as a neutronsource monitoring system.

[0052] There are other applications and processing procedures of theinvention that will become apparent to those of ordinary skill in theart.

[0053] While the foregoing disclosure is directed toward the preferredembodiments of the invention, the scope of the invention is defined bythe claims, which follow.

What is claimed is:
 1. A borehole instrument responsive to radiationfrom borehole environs, the instrument comprising a YAP scintillator. 2.The borehole instrument of claim 1 further comprising a light responsivemeans optically coupled to said YAP scintillator, wherein output of saidlight responsive means is a function of scintillation intensity withinsaid YAP scintillator.
 3. The borehole instrument of claim 2 furthercomprising means for transforming said output from said light responsivemeans into a characteristic of radiation reacting with said YAPscintillator.
 4. The borehole instrument of claim 2 wherein said lightresponsive means comprises a photomultiplier tube.
 5. The boreholeinstrument of claim 3 wherein said characteristic is intensity ofradiation reacting with said YAP scintillator.
 6. The boreholeinstrument of claim 3 wherein said characteristic is energy of radiationreacting with said YAP scintillator.
 7. The borehole instrument of claim1 wherein said radiation is gamma radiation.
 8. The borehole instrumentof claim 7 wherein said gamma radiation is inelastic scatter gammaradiation.
 9. The borehole instrument of claim 3 further comprisingmeans for transforming said characteristic of radiation into a propertyof borehole environs in which said borehole instrument is disposed. 10.The borehole instrument of claim 9 wherein said property is a propertyof material penetrated by said borehole.
 11. The borehole instrument ofclaim 9 wherein said property is a property of materials within and inthe immediate vicinity of said borehole.
 12. The borehole instrument ofclaim 1 wherein said instrument is conveyed along a borehole by awireline.
 13. The borehole instrument of claim 1 wherein said instrumentis conveyed along a borehole by a drill string.
 14. A method formeasuring radiation in a borehole environment, comprising the step ofanalyzing scintillations generated by interaction of said radiation fromborehole environs with a YAP scintillator.
 15. The method of claim 14wherein said analysis of scintillations comprises the steps of: (a)optically coupling a light responsive means to said YAP scintillator;(b) measuring output of said light responsive means; and (c) relatingsaid output to a characteristic of said radiation.
 16. The method ofclaim 15 wherein said characteristic is energy of said radiation. 17.The method of claim 15 wherein said characteristic is intensity of saidradiation.
 18. The method of claim 14 wherein said radiation is gammaradiation.
 19. The method of claim 14 wherein said YAP scintillator isconveyed along a borehole by means of a wireline.
 20. The method ofclaim 14 wherein said YAP scintillator is conveyed along a borehole bymeans of a drill string.
 21. A borehole logging system comprising: (a) aradiation source; and (b) a radiation detector responsive to radiationinduced in borehole environs by said radiation source and comprising aYAP scintillator, a light responsive means coupled to said scintillator,and circuitry connected to said light responsive means for processingoutput from said scintillator, wherein (c) said radiation detectorprocesses detected scintillations with minimal light decay time fromsaid scintillator to minimize pulse pile-up in said radiation detector.22. The system of claim 21 wherein said radiation source is a neutronsource.
 23. The system of claim 22 wherein said neutron source is apulsed neutron source.
 24. The system of claim 23 wherein said radiationdetector is operated during pulses from said pulsed neutron source. 25.The system of claim 23 wherein said radiation detector is operatedduring quiescent periods between pulses from said pulsed neutron source.26. The system of claim 21 wherein said radiation detector is a gammaray detector.
 27. The system of claim 21 further comprising: (a) ahousing in which said radiation source and said radiation detector aredisposed; and (b) a wireline for conveying said housing along aborehole.
 28. The system of claim 21 further comprising: (a) a housingin which said radiation source and said radiation detector are disposed;and (b) a borehole drilling string for conveying said housing along aborehole.
 29. The system of claim 21 further comprising means fortransforming response of said radiation detector into a boreholeenvirons parameter of interest.
 30. A method for measuring acharacteristic of environs penetrated by a borehole comprising the stepsof: (a) irradiating said environs with a source of radiation therebyinducing radiation within said environs; and (b) measuring said inducedradiation with a radiation detector comprising a YAP scintillator, alight responsive means coupled to said scintillator, and circuitryconnected to said light responsive means for processing output from saidscintillator, wherein (c) said radiation detector processes detectedscintillations with minimal light decay time from said scintillator tominimize pulse pile-up in said radiation detector.
 31. The method ofclaim 30 wherein said source of radiation is a neutron source.
 32. Themethod of claim 31 comprising the additional step of pulsing saidneutron source.
 33. The method of claim 32 comprising the additionalstep of operating said radiation detector during pulses from saidpulsing neutron source.
 34. The method of claim 32 comprising theadditional step of operating said radiation detector during quiescentperiods between pulses from said pulsing neutron source.
 35. The methodof claim 30 wherein said radiation detector is a gamma ray detector. 36.The method of claim 30 comprising the additional steps of: (a) disposingsaid source of radiation and said radiation detector in a housing; and(b) conveying said housing along a borehole by means of a wireline. 37.The method of claim 30 comprising the additional steps of: (a) disposingsaid source of radiation and said radiation detector in a housing; and(b) conveying said housing along a borehole by means of a drill string.38. The method of claim 30 further comprising the step of transformingresponse of said radiation detector into said characteristic ofenvirons.